Calcium ions play a fundamental role in the physiology and biochemistry of organisms and of cells. The entry of calcium into cells through ion channels mediates a variety of cellular and physiological responses, including gene expression, signal transduction, neurotransmitter release, muscle contraction and hormone secretion. Ion channels are classified by gating, or what opens and closes the channel to the flux of ions. Voltage-gated ion channels open or close depending on the voltage gradient across the plasma membrane, whereas ligand-gated ion channels open or close depending on the binding of ligands to the channel. The classification of voltage-gated calcium channels divides them into three groups: (i) high voltage-activated channels, which include L-, N-, P- and Q-type channels; (ii) intermediate voltage-activated R-type channels; and (iii) low voltage activated T-type channels.
The N-type calcium channel is distributed mainly in central and peripheral neurons, being localized primarily to presynaptic nerve terminals. This channel regulates the calcium flux required for depolarization-evoked release of neurotransmitters from synaptic endings. The transmission of pain signals from the periphery to the central nervous system (CNS) is mediated, inter alia, by N-type calcium channels located in the spinal cord. Inhibition of the N-type calcium channel in the superficial dorsal horn leads to a decrease in membrane excitability and neurotransmitter release, resulting in pain relief. In addition, knock-out mice lacking the N-type calcium channel exhibit reduced nociceptive behaviors in animal models of pain.
N-type calcium channels have been shown to mediate the development and maintenance of the neuronal sensitization processes associated with neuropathic pain and therefore provide attractive targets for the development of analgesic drugs. Three N-type calcium channel modulators are currently approved for the treatment of pain: ω-conotoxin MVIIA (ziconotide), marketed as Prialt®, potently and selectively blocks the N-type calcium channel and is indicated for the management of severe chronic pain; gabapentin, marketed as Neurontin®, and pregabalin, marketed as Lyrica®, bind with high affinity to the α2δ subunit of the N-type calcium channel and are indicated for the treatment of fibromyalgia, diabetic nerve pain and/or post-herpetic neuralgia pain.